A downdraft gasifier works because one hot zone pays for another: oxidising carbon to carbon dioxide releases 393.5 kJ per mole, and reducing carbon dioxide back to carbon monoxide over glowing charcoal absorbs 172.5 kJ per mole, so roughly 0.44 mol of carbon must burn for every mole reduced when heat losses are ignored. The reduction bed only makes combustible gas while it stays hot enough, around 800 °C and above, and it cools itself with every reaction it carries out.
Part of Biomass Gasification
The thermochemistry of the four zones in a downdraft gasifier: which reactions release heat, which absorb it, and why the balance fixes the gas composition.
The Heat Budget Is Set By Six Reactions
Each enthalpy below follows from standard enthalpies of formation at 25 °C: CO₂ −393.5, CO −110.5, H₂O (as vapour) −241.8 and CH₄ −74.8 kJ/mol, with elements at zero. These are standard thermochemical data, not measurements from a gasifier.
| Reaction | Zone | ΔH, kJ per mol |
|---|---|---|
| C + O₂ → CO₂ | oxidation | −393.5 (releases heat) |
| C + ½O₂ → CO | oxidation | −110.5 (releases heat) |
| C + CO₂ → 2CO (Boudouard) | reduction | +172.5 (absorbs heat) |
| C + H₂O → CO + H₂ (water-gas) | reduction | +131.3 (absorbs heat) |
| CO + H₂O → CO₂ + H₂ (shift) | reduction | −41.2 (releases heat) |
| C + 2H₂ → CH₄ | reduction | −74.8 (releases heat) |
Derivation of two entries: Boudouard is 2(−110.5) − (−393.5) = +172.5. Water-gas is (−110.5) − (−241.8) = +131.3. Pyrolysis and drying are not in the table because they have no single reaction: drying costs about 2.44 MJ per kilogram of water evaporated, and pyrolysis is a net heat-absorbing step whose heat reaches the fuel by radiation and conduction upward from the oxidation zone just below it.
Worked Example: How Much Carbon Must Burn To Feed The Bed
Take 1 mol of carbon reduced by the Boudouard reaction, which absorbs 172.5 kJ.
- If the heat comes from burning carbon completely to CO₂: 172.5 ÷ 393.5 = 0.44 mol of carbon burned per mole reduced.
- If it comes from burning only to CO: 172.5 ÷ 110.5 = 1.56 mol of carbon burned per mole reduced.
So the oxidation zone has to run mostly to carbon dioxide, which is what a hot, well-supplied throat does, and the reduction bed then converts that CO₂. This ignores sensible heat in the gas and losses through the walls, so a real bed needs more. The water-gas reaction is cheaper to run (131.3 kJ per mole) but uses steam, so wet fuel feeds it; it also takes heat from the bed, which is the first mechanism by which wet fuel lowers the gas quality.
Temperature Decides Whether The Bed Makes CO Or Leaves CO₂
The Boudouard equilibrium at 1 atm is temperature-dependent: recalled textbook values put it at about half CO and half CO₂ near 700 °C and almost entirely CO by 900 °C, and mostly CO₂ below about 600 °C. Because both reduction reactions absorb heat, the gas cools as it crosses the bed, and the lower part of a thick bed runs cold and stops reacting. The carrier’s reduction range of 600-1,000 °C therefore spans the useful region (above about 800 °C) and the dead one. A bed that is too short leaves unreacted CO₂ and steam; one that is too long wastes height without gas.
Air Is Rationed To About A Third Of Complete Combustion
Take the gas yield of about 2.5 m³ per kilogram of dry wood derived in Energy Content of Wood Gas and nitrogen at 50-55% of the gas (carrier range). The nitrogen came in with the air, which is 79% nitrogen:
- Nitrogen per kg of dry wood: 2.5 × 0.525 = 1.31 m³.
- Air per kg of dry wood: 1.31 ÷ 0.79 = 1.66 m³.
- Complete combustion of dry wood needs about 6 kg of air per kg of fuel (recalled textbook figure), which is 6 ÷ 1.293 = 4.6 m³.
- Ratio: 1.66 ÷ 4.6 = 0.36, about a third.
That fraction is the only control on the whole process: more air raises the oxidation zone’s temperature and the CO₂ fraction; less air starves it of heat and leaves tar.
Why Downdraft Flow Cracks Tar
Pyrolysis vapours, which include the tar, leave the pyrolysis zone and must pass through the oxidation zone at its narrowest point before they leave the vessel. Thermal cracking of tar needs sustained temperatures around 1,000 °C and above (recalled), and the throat is where those temperatures exist. In an updraft the gas leaves through the pyrolysis zone instead, with no hot zone downstream, which is why that design produces tarry gas (Updraft Gasifier). The geometry that concentrates the heat is in Hearth Constriction.
Common Mistakes
- Assuming reduction is exothermic. Both main reduction reactions absorb heat. A bed that is not fed by the oxidation zone goes out.
- Quoting zone temperatures as fixed. They overlap and move with load, moisture and air rate.
- Using the equilibrium as a prediction. Real beds do not reach equilibrium; the table gives the direction, not the composition.
Safety
The reduction bed is where carbon monoxide is made
The gas leaving it is about 18-22% carbon monoxide by volume, odourless and colourless, and it is hot, so it reaches the lighting port, ash door and hopper lid as readily as the outlet if any of them leaks. Never run a gasifier indoors or in an enclosed space. Opening a hot bed to air can light the gas inside it. Do not use the smell of tar as a guide to the air: it does not track carbon monoxide. See Carbon Monoxide Risks.
What’s Next
- Learn next: Syngas Composition — what these reactions produce, and what shifts it
- Combine with: Hearth Constriction — the geometry that holds the oxidation zone hot